Oil and gas wellbore fluids (wellbore fluids) are fluids that are used downhole during the operations performed for the purpose of exploration or extraction of gas or oil from subterranean formations.
Drilling fluids, which are also called drilling muds, are complex mixtures of chemicals used in drilling operations for the production of hydrocarbons and natural gas from subterranean reservoirs.
Drilling fluids are pumped into the drilling shaft and exit from the drilling bit through openings. The drilling fluids return to the surface through the annulus between the outside of the drilling shaft and the bore hole wall.
Drilling fluids perform a number of functions. Exemplary of these functions are carrying drill cuttings up to the surface and suspending them when the fluid circulation is stopped; creating a filter cake on the bore hole walls to reduce permeability; cooling and lubricating the drill bit; creating hydrostatic pressure to avoid uncontrolled blow outs and to help supporting the weight of the bore hole walls; and acting as lubricant between the drill bit, the drill string and the bore hole walls. In general, the use of drilling fluids alone is not sufficient to reduce friction substantially, especially if horizontal and highly deviated wells are considered. Thus, in general, a suitable lubricant additive has to be added to the drilling fluid.
Drilling fluids may be of the water based type or of the oil based type.
Water based drilling fluids are environmentally safer and less costly compared to oil based fluids, but the latter usually present higher lubricating performances. In both cases, however, lubricant additives are regularly employed.
At the end of the drilling operations, the drilling mud is usually replaced with a completion fluid (CF). CF are fluids employed in completion operations, i.e. in the whole series of actions/processes required for making the well ready for production. CF may comprise brines free from undissolved solids (based on chlorides, bromides and formates), as well as any fluid with proper density and flow characteristics, chemically compatible with the formation.
CF normally require reliable and efficient lubricants for lowering the torque and drag due to the frictional forces within the well which may induce the sticking of downhole tubulars and coils.
In wellbore operations, lubrication of coiled tubing (CT) is also of primary significance. CT usually refers to long metal pipes rolled up on a large reel. CT can be used, by way of example, for interventions into gas and oil wells as well as production tubing in depleted gas wells. CT can thus be employed for operations analogous to wirelining where it offers some advantages. Among them, the coil can be pushed into the hole (rather than relying on gravity) and chemicals can be pumped through the coil.
Coiled tubing fluids (CTF) are used to pose the coiled tubing and require the addition of efficient lubricants too.
Many lubricants for applications into wellbore fluids have been described, including solids, such as plastic or glass beads, nanoparticles and graphite, or liquids, such as oils, synthetic fluids, glycols, modified vegetable oils, fatty acid soaps and surfactants.
By way of example, U.S. Pat. No. 5,618,780 relates to an optimized lubricating composition including, in useful amounts, an ester and a fatty acid.
U.S. Pat. No. 8,071,510 describes a lubricant based on a water-soluble or water-dispersible salt of a sulfonated (sulfated) vegetable oil or a derivative thereof, such as a sulfonated (sulfated) castor oil.
In U.S. Pat. No. 8,148,305, the use of oligoglycerol fatty acid esters as additives in water-based drilling mud compositions for improving the lubricating action of these mud systems is described.
Microemulsions of oils and fatty esters, which are typical ingredients of lubricants, are well known and described in the patent literature, as adjuvants for pharmaceutical applications (by way of example in U.S. Pat. No. 6,451,325 and U.S. Pat. No. 6,299,884), for agrochemical applications (by way of example in U.S. Pat. No. 8,282,950) and also for use in the oil industry (by way of example in US 2013/0261033 and U.S. Pat. No. 7,902,123).
It has now been found that water in oil microemulsions containing insoluble particles of metal hydroxides and/or metal oxides in the inner aqueous phase are very efficient in increasing the lubricity of wellbore fluids, thus reducing the coefficient of friction of drilling, completion and coiled tubing fluids without significantly altering the mud rheology. The microemulsions are particularly effective when the insoluble particles of metal hydroxides and metal oxides have been synthesized from their salt precursors in the water droplets of the microemulsion and are nanosized, i.e. are between 1 nm and 100 nm in size; actually, it has been observed that nanoparticles that have been synthetised in microemulsion show a surprisingly higher lubricating effect in well fluids.
The use of nanoparticles as lubricity additives in well fluids is already known. IADC/SPE 161899 reports that a lubricating material containing nanoscale solid corpuscles was used in drilling fluids in Daqing Oil Field.
CA 2586832 discloses a drilling bit grease that comprises form 0.1 to 10% by weight of a nanomaterial.
CN 1699500 reports that the addition of pre-prepared nanoparticles to emulsion lubricants for drilling fluids enhance their stability.
In WO 2013/116921 the lubricant additive for drilling fluids is based on nanoparticles that are prepared in situ in the drilling fluid or ex situ, by high shear mixing two aqueous solutions containing the nanoparticles precursors. Nonetheless, as far as the Applicant knows, the use of microemulsions containing insoluble particles of metal hydroxides and/or metal oxides in the dispersed (or inner) aqueous phase, for increasing the lubricity of drilling or completion fluids, has never been reported.